1. Field of the Invention
The present invention relates to methods of making thin film and multilayer coatings, and more specifically, it relates to a method for the production of axially symmetric, graded and ungraded thickness thin film and multilayer coatings that avoids the use of apertures or masks to tailor the deposition profile.
2. Description of Related Art
The ability to provide spatially varying thin film and multilayer coatings is essential for several advanced technologies. In particular, it is critical to developing optical fabrication techniques capable of fulfilling the requirements of soft x-ray or extreme ultra violet projection lithography (SXPL or EUVL). Large field EUVL imaging systems will require multiple aspheric optical elements figured to subnanometer accuracy. In order to achieve the optimum imaging performance, graded period multilayer (ML) coatings must be deposited on these aspheric surfaces with gradients of 1 part in 10.sup.9. This corresponds to a ML period variation of 1 .ANG. across a 10 cm diameter substrate. This is a demanding requirement since the ML coating distribution is, in general, a complicated function of source and substrate variables.
For example, consider physical vapor deposition onto a stationary substrate. During time t, the substrate surface will acquire a coating thickness T at a position (x,y,z) given by ##EQU1## Here, .PHI. is the flux of depositing species, S is the sticking coefficient and G accounts for effects associated with the substrate geometry. In general all of these quantities are position and time dependent. Frequently, the sticking coefficient will only exhibit a weak spatial and temporal dependence and is assumed to be constant, S (x,y,z,t')=S. Also, provided that the deposited coating(s) produce only slight changes in the substrate shape, G is time independent and (1.1) reduces to ##EQU2## Only in the case where .PHI.(x,y,z,t)=.OMEGA.(x,y,z).GAMMA.(t) are variations arising from spatial influences and temporal fluctuations separable, giving ##EQU3## Suppose that a uniform coating is desired. Inserting a mask with an aperture function A(x,y,z)=C.sub.0 S.sup.-1 G.sup.-1 (x,y,z).OMEGA..sup.-1 (x,y,z) between the source and substrate will yield a film thickness given by ##EQU4## Here A(x,y,z) depends upon both source and substrate characteristics. Note that a different aperture is required for each source-substrate combination even in this simple case. In most systems the spatial and temporal fluctuations that arise in the deposition process are not separable. In general, provision must be made for complex relative motion(s) of the source(s) and substrate(s) to provide for spatial and temporal averaging of the deposition process, inserting masks to approach the desired coating profile(s). At best this is a tedious and inefficient process that is inappropriate for a robust manufacturing technique. In fact it is not clear that these techniques have the capability of accessing controlled deposition profiles with an accuracy of 1 part in 10.sup.9.